A method of analyzing a fluid sample by XRF includes the steps of: depositing the fluid sample onto a substrate, exposing the sample and the substrate to an x-ray emission, generating a first analytical signal responsive to the x-ray emission, providing an operable first reference material having a first extended position above the substrate and in communication with the x-ray emission, periodically extending the first reference material to its first extended position, generating a first calibration signal, providing an operable second reference material having a first extended position below the substrate and in communication with the x-ray emission, and periodically extending the second reference material to its first extended position and generating at least one second calibration signal. The first and second calibration signals are compared with predetermined values. The first and second reference materials also have second retracted positions.